This invention pertains to a highly sensitive method of detecting small deformation or movement of the windings of a high voltage transformer over a broader frequency range (1 kHz to 20 MHz) than prior art techniques (which are typically restricted a frequency range of about 1 kHz to 3 MHz), without requiring external leads or removal of the transformer from service.
High voltage power transformers (i.e. 50 or 60 Hz oil-filled transformers with primary voltages ranging from 69 kV to 750 kV and ratings from 5 kVA to over 500 MVA) are the most expensive pieces of equipment in a typical interconnecting power system. Keeping such transformers in service is critical to the operation of the power system. Transformers normally operate quite reliably over their typical 30 year design life spans. However, transformer failures do sometimes occur, with consequential severe impacts including loss of service and resultant loss of revenue; equipment damage which can be very expensive to repair or replace; and, potentially serious explosion, fire or other safety hazards to utility operations personnel.
It is well known that movement, looseness, deformation or distortion of a transformer""s windings can lead to catastrophic electrical or mechanical failure of the transformer. A transformer""s windings may move, etc. if the transformer is subjected to an electrical short circuit, which is not uncommon. Furthermore, as a transformer ages, the paper insulation material applied to the transformer""s windings tends to shrink, thereby reducing the overall winding clamping pressure and allowing the windings to move, etc. It is very difficult to reliably detect small movement, etc. of a transformer""s windings, yet early and reliable detection of such conditions is highly desirable in order to avoid the aforementioned catastrophic consequences.
The conventional prior art method of detecting movement, etc. of a transformer""s windings is to remove the transformer from service, open the transformer, and visually inspect the windings. The transformers in question are normally oil-filled, so it is necessary to drain the oil to facilitate inspection, and replace the oil after inspection. This is very time consuming, requires a long transformer outage interval and is quite expensive.
Another prior art method of detecting movement, etc. of a transformer""s windings is the so-called xe2x80x9ctransformer short circuit impedance testxe2x80x9d. However, this technique requires a relatively large AC power source which is usually unavailable at the site of the transformer to be tested. Moreover, measurements of a transformer""s short circuit impedance at the frequencies of interest (i.e. the transformer""s normal 50 or 60 Hz operating frequency) are insufficiently sensitive to detect small winding deformation or movement, etc. and even less sensitive to detection of winding looseness.
The prior art has evolved an alternative xe2x80x9cfrequency response analysisxe2x80x9d or xe2x80x9clow voltage impulsexe2x80x9d testing technique, which is considerably more sensitive to the detection of transformer winding deformation, etc. than the short circuit impedance test. This alternative test can be performed without opening the transformer and without a large AC power source. As shown in FIG. 1, transformer 10 is removed from service and a signal source 12 such as a recurrent surge generator is electrically connected to one of the transformer""s input windings 14. A current shunt 16 is electrically connected to the transformer""s input winding (as shown) or another winding (not shown). A recording device 18 such as a digital oscilloscope is electrically connected to the input winding and the output of the current shunt. Actuation of signal source 12 applies a test signal to input winding 14, producing a current in the input winding to which current shunt 16 is connected or, if current shunt 16 is connected in the output winding, a capacitively coupled current signal. Alternatively the current shunt is disconnected and the voltage coupled into the output winding is measured. Both the applied signal and the resultant capacitively coupled signal are recorded by recording device 18, and the data so obtained is then used to calculate a transfer function for the transformer, in conventional fashion. The data and/or transfer function are retained for future comparison with additional data and/or transfer function(s) obtained during subsequent testing of the same transformer under identical test conditions. The comparison can also be done with another transformer of identical design; although such comparisons are not as accurate as comparisons of test results obtained for the same transformer over time. The objective is to detect differences between transfer functions obtained at different testing times, with such differences possibly being indicative of transformer winding movement, etc.
Either one of two different test methods can be used to perform the frequency response analysis test to obtain the desired transfer function; namely, the swept frequency test method or the pulse test method. A swept frequency test is performed by applying a variable frequency voltage or a white noise voltage input signal to the transformer""s high voltage winding terminal (normally the input winding terminal) and recording the output response signal produced in another winding terminal (normally the output winding terminal) of the transformer. The output signal divided by the input signal for each test frequency yields the transformer""s transfer function as a function of frequency. A pulse test is performed by applying an input pulse signal containing energy at all frequencies of interest to the input winding, and recording the output response signal produced in another winding (normally the output winding). The recorded data (applied voltage input and capacitively coupled current output signal) are then each subjected to a Fourier transform. The Fourier transform of the capacitively coupled current output signal is divided by the Fourier transform of the applied voltage input signal to obtain the transformer""s transfer function as a function of frequency.
The input and output signals can be measured on any combination of input and output terminals of the transformer. All combinations will have some sensitivity to transformer winding deformation, etc. Normally, the most sensitive measurement is obtained using the high voltage winding for the input signal and the low voltage winding for the output signal. Other combinations are sometimes used e.g. input to high voltage winding and output on neutral to obtain more information, or due to transformer design limitations.
A transformer""s transfer function is independent of the applied signal source, but dependent upon the transformer""s internal structure. More particularly, a combination of factors including winding inductances and winding capacitances (inter-turn capacitance, interwinding capacitance, winding-to-tank capacitance, etc.) determine the transfer function. Any movement, looseness, deformation or distortion of a transformer""s windings can change the transformer""s capacitance characteristics, thereby changing the transformer""s transfer function. By carefully comparing a transfer function obtained via testing while a transformer is in a known satisfactory operating condition with a transfer function obtained via later testing of the same transformer, one may detect changes indicative of transformer winding movement, etc. The higher the maximum frequency of the transfer function, the more sensitive the test is to winding movement.
FIG. 2 provides further details of the prior art frequency response analysis test using the pulse test method. A signal source such as a recurrent surge generator 12A is used to apply a pulse of about 300 volts to each one of transformer 10A""s input winding terminals in turn. The transformer""s neutral terminal H0/X0 and the corresponding tertiary winding terminals Y1, Y2, Y3 are in turn grounded through current shunt 16A. This represents the measurements normally done on an auto-transformer having a tertiary winding. There are many other transformer winding configurations that can be tested (e.g. single phase dual winding, three phase dual winding, etc.). Other test configurations can be used for transformer 10A, but the six measurements described above and tabulated below are the most common:
The applied voltage CR01 and the voltage CR02 across current shunt 16A (representative of the winding current or the current which is capacitively coupled into the secondary winding) are recorded on recording device (digital oscilloscope) 18.
Besides the aforementioned dependence upon internal structure, a transformer""s transfer function is also critically dependent upon the physical characteristics of the leads used to connect the applied signal source, current shunt and recording instrumentation to the transformer. Therefore, to facilitate comparison of transfer functions obtained via tests conducted at different times, a careful record of the test measurement setup (FIG. 3) is retained, together with accurate measurements of: the length and wire gauge of each of leads L1 through L6; the model, length, voltage division ratio, and input impedance of voltage probe C1 (typically provided as an accessory to digital oscilloscope 18); the length, impedance and cable type of coaxial cables C2, C3; and, the model and impedance of current shunt 16A. In subsequent testing of the same transformer an identical test measurement setup, identical lead lengths, etc. must be employed to facilitate meaningful comparison of transfer functions obtained via tests conducted at different times. It is noteworthy that the lengths of leads L1-L6 and coaxial cables C2, C3 depend upon the size of the transformer being tested, and can be 20 metres or longer. (In order to obtain the H0/X0 measurement, the FIG. 3 setup is modified by disconnecting lead L1 and connecting lead L5 to the input of current shunt 16A.
Any test which employs external leads (i.e. leads L1 to L6 as shown in FIG. 3) is subject to a frequency range limitation (about 3 MHz for the pulse test method) due to the leads"" impedance and due to the sensitivity of lead positioning at higher signal frequencies. More particularly, at high frequencies the lengthy external leads have high impedance resulting in significant signal attenuation and noise; and, the leads"" positions (which can not be exactly duplicated for different tests) affects the test results. Even a slight change in lead position affects high frequency test results. Due to the physical size of the transformer and its bushings, it is not possible to shorten the leads to improve the frequency response. Consequently, prior art frequency response analysis techniques are comparatively useless at higher signal frequencies.
This is illustrated by FIGS. 8 and 9, which reveal that neither linear nor dB plots are adequate to enable useful discrimination between transfer functions derived from data measurements obtained during xe2x80x9cnormalxe2x80x9d operating conditions and after subjecting the same transformer""s windings to minor movement. At higher frequencies the two plots appear to substantially overlap, which could cause inaccurate interpretation of the solid line transfer function as representative of acceptably normal operation, notwithstanding the fact that the data from which the solid line transfer function was obtained reflects winding movement as aforesaid.
The invention avoids the above disadvantages by providing a measurement technique which requires neither external leads nor removal of the transformer from service, is highly sensitive to the effects of transformer winding movement, etc. and readily facilitates transfer function discrimination of such effects at high signal frequencies.
The invention facilitates detection of transformer winding movement by providing an input sensor inside the transformer on an input winding lead and providing an output sensor inside the transformer on an output winding lead. This eliminates the frequency limitation imposed by the lead lengths required for prior art external measurement, making it possible to obtain useful measurements at much higher frequencies (20 MHz) than with the prior art techniques. Both sensors are electrically connected to a signal recorder provided outside the transformer via high bandwidth cables which extend through the transformer""s casing.
During a first test interval and while the transformer is in a known satisfactory operating condition, a first test signal is applied to the input winding lead while recording on the signal recorder (i) a first output signal, produced by the input sensor, representative of the first test signal, and (ii) a second output signal, produced by the output sensor, representative of the first test signal after coupling of the first test signal through a winding of the transformer to the output winding lead. During a second test interval and while the transformer is in an unknown operating condition, a second test signal is applied to the input winding lead while recording on the signal recorder (i) a third output signal, produced by the input sensor, representative of the second test signal, and (ii) a fourth output signal, produced by the output sensor, representative of the second test signal after coupling of the second test signal through a winding of the transformer to the output winding lead. The first and second signals are combined to produce a first transfer function representative of operation of the transformer during the first test interval; and, the third and fourth signals are combined to produce a second transfer function representative of operation of the transformer during the second test interval. The first and second transfer functions are then compared to detect differences between them.
The test signals preferably include energy at signal frequencies throughout the range of about 1 KHz to about 20 MHz. The signal source that produces the test signals may be a recurrent surge generator provided outside the transformer. Alternatively, the test signal may be a voltage transient produced by disconnecting a circuit breaker in the power system energizing the transformer; or, the test signal may be received through antenna coupling to a power transmission line in the power system.
The output sensor may be a capacitive voltage sensor, an (optical or electrical) electric field sensor, or a (optical or electrical) high frequency current transformer, a Rogowski coil, an optical current transducer, or any other sensor capable of measuring the electric field, voltage, current or magnetic field at the internal winding leads.